What makes a film a so-called Cult Classic? Like many things, this can be pretty subjective, but in my opinion, cult films successfully "push the envelope."
I love the definition I found on http://www.filmsite.org/cultfilms.html:
"Cult Films have limited but very special appeal. Cult films are usually strange, quirky, offbeat, eccentric, oddball, or surreal, with outrageous, weird, unique and cartoony characters or plots, and garish sets. They are often considered controversial because they step outside standard narrative and technical conventions. They can be very stylized, and they are often flawed or unusual in some striking way."
Here's what Wikipedia has to say about them: http://en.wikipedia.org/wiki/Cult_film
The first film that comes to my mind is Rocky Horror Picture Show. Have you ever been to a showing, where you dress up like a transvestite, bring a piece of unbuttered toast, and sing along? If not, this is for you, my dear: http://www.rockyhorror.com/participation/virgins.php
I found out something very interesting at filmsite.org. Did you know that the re-released version of The Sound of Music also had a sing-along audience, and they dressed up as nuns, Nazis and goatheads? What a trip!
One of my all-time favorite films is Life is Beautiful, and in researching this post, I discovered that some consider it a Cult Classic. Office Space, Big Lebowski (which I only recently got my hands on), and Heathers are a few of my favorites. One I didn't enjoy in the least (but might have to rewatch someday as an adult) is A Clockwork Orange. And sadly, I haven't seen a single one on this Top 10 list: http://worldfilm.about.com/od/cultfilms/tp/cultdvds.htm
What is YOUR favorite Cult Film? Here's a list of the Top 50 if you need a little help: http://5mtl.com/ft/culttop.htm